The present invention relates to tanks designed to hold gas or volatile liquids, and more particularly, is designed to improve tanks which store pressurized gas, either in conjunction with a natural gas supply system, or an anaerobic wastewater treatment digester.
In the case of anaerobic digesters, gases, typically methane and carbon dioxide, are given off and collected to be used as the fuel for heating the sludge mixture. Traditional storage tanks employ a floating gas holder positioned above the sludge which collects the gas and provides a controllable downward force thereon, pressurizing the gas and making it immediately usable by the sludge heating equipment.
These tank facilities must meet environmental limitations concerning the discharge of gases into the atmosphere. Careful monitoring and control of the gases produced during the anaerobic process is essential, since the gases frequently are explosive when mixed with ambient atmospheric air. The conventional floating gas holder, while workable in theory, is subject to corrosion, freezing, tipping and gas leakages, all of which substantially interfere with its effective operation to properly control the gas within the tank.
Attempts have been made to solve the deficiencies of the common floating gas holder by providing the digester with a fixed outer cover and a flexible, pressurized gas retention membrane underneath. U.S. Pat. No. 4,060,175 to Rysgaard, discloses such a membrane connected to a vertically slidable stack equipped with pressure release and flame arrester system. A flowable granular ballast material placed upon the membrane near the tank wall serves as the pressure regulating mechanism.
In operation, the Rysgaard systems have proved to be impractical, largely due to the unwieldy nature of the ballast used to exert pressure on the membrane, as well as the inability of the flexible membrane to retain the ballast and simultaneously maintain a gas-proof seal.
Commonly-assigned U.S. Pat. No. 4,437,987 to Thornton, et al. discloses a digester cover with a fixed outer cover and a fixed center gas well, to which inner and outer flexible membranes are affixed. The inner membrane collects the digester gas. The cavity between the inner and outer membranes is filled with pressurized air as the gas pressure regulating means. The Thornton, et al. system has overcome most of the drawbacks of the Rysgaard and floating gas well designs, but the cost of constructing the fixed outer dome has rendered the system noncompetitive as compared to conventional floating gas holder designs.
The fixed cover and gas well assembly is necessary to provide the following features: a means of mounting pressure/vacuum release systems; a support means to prevent a deflated membrane from falling into the sludge or being impaled upon an internal projection such as a pipe; a means of introducing air between the membranes without penetrating either membrane; and the provision of access to the interior of the tank for periodic cleaning.
Thus, it is an object of the present invention to provide a digester gas collection system with a low-cost, yet structurally sound gas well support structure and a flotaing gas well with maintenance access means.
It is a further object of the present invention to provide a flexible membrane gas collection system which exerts minimal loading on the gas well support structure.